The present invention relates to valve metals and electrolytic capacitors using the valve metals as well as methods of making the valve metals and the capacitors. More particularly, the present invention relates to sintered valve metal materials and sintered bodies such as capacitors having high capacitance made from the sintered valve metal material.
Capacitors in general, and valve metal capacitors in particular, have been a major contributor to the miniaturization of electronic circuitry. Valve metal capacitors typically are manufactured by compressing valve metal powder to form a pellet, sintering the pellet in a furnace to form a porous tantalum body (electrode), and then subjecting the porous body to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body. Valve metal powder which is suitably employed in an anode electrode of a solid electrolytic capacitor may include, for example, powder of niobium, tantalum, titanium, tungsten, and/or molybdenum.
The performance characteristics of capacitors or electrodes formed from capacitor grade powders are expressed in terms of specific charge and electrical current leakage. The specific charge is a measure of electrical charge capacity of the capacitor and is usually proportional to the surface area of the powder as a sintered and anodized pellet. The electrical leakage is an indication of how well the capacitor holds the specific charge. Capacitors with improved electrical leakage characteristics are recognized as having higher reliability.
Development of valve metal powders suitable for making capacitors has resulted from efforts by both capacitor producers and valve metal processors to delineate the characteristics required for capacitor-grade powder for it to best serve in the production of quality capacitors. Such characteristics include specific surface area, purity, shrinkage, pressability, and the like. The powder preferably provides an adequate electrode surface area when formed into a porous body and sintered. The μFV/g of tantalum capacitors can be related to the specific surface area of the sintered porous body produced by sintering a valve metal powder pellet. The specific surface area of valve metal powder can be related to the maximum μFV/g attainable in the sintered porous body.
Purity of the powder is an important consideration. Metallic and non-metallic contamination tends to degrade the dielectric oxide film in valve metal capacitors. While high sintering temperatures serve to remove some volatile contaminants, high temperatures also tend to shrink the porous body, reducing its net specific surface area and thus the capacitance of the capacitor produced. Minimizing the loss of specific surface area under sintering conditions, i.e., minimizing shrinkage, is desirable to produce high μFV/g valve metal capacitors.
As noted, the μFV/g of a valve metal pellet can be affected by the specific surface area of the sintered powder. Greater net surface area can be achieved, of course, by increasing the quantity (grams) of powder per pellet; but, cost and size considerations have dictated that development be focused on means to increase the specific surface area of valve metal powder.
Sintering is the bonding of powder compacts by the application of heat to enable one or more of several mechanisms of atom movement that eliminate or reduce the number of contact interfaces between particles. The mechanisms that account for the sintering process are known and have been described, for example, in “Sintering Theory and Practice,” R. M. German, J. Wiley and Sons, New York (1996), which is incorporated herein in its entirety by reference. The sintering mechanisms include viscous flow, liquid phase solution-precipitation, bulk diffusion, surface diffusion, and evaporation-condensation. Sintering mechanisms generally causes densification thus promoting shrinkage in the sintered body. However, it has been reported, for example in “Surfactant Aided Dispersion of Nanoparticular Suspension of Welding Fume,” S. Adelman, at www.mit.edu/˜sca23/simonadelman/surfactant.html, incorporated herein by reference, that the sintering mechanisms, surface diffusion and evaporation/condensation, do not lead to densification of the sintered material.
Sintering is typically carried out at high temperature (e.g., 1500-2000° C.) under vacuum. Sintering causes the individual powder particles to join together to form a porous structure. The structure is preferably of high mechanical strength and density, but is also preferably highly porous, exhibiting a large internal surface area. Sintered bodies that are subjected to sintering for excessive times or at high temperatures tend to form sintered material fused together too much, resulting in an anode formed therefrom having low specific surface area and low capacitance. Similarly, if the anodes are sintered for an insufficient time, or the furnace temperature is insufficiently low, the mechanical strength can be insufficient even though the capacitance is high.
Oxidation of valve metal material can occur at various stages in the production or processing of the valve metal material by various active or passive processes. For example, the valve metal can gain an oxide layer due to surface reaction with air under ambient or other conditions. The oxygen content of valve metal material can be controlled, for example, by deoxidizing the valve metal at one or more stages in its processing. The deoxidation is typically achieved by introducing an oxygen getter to the valve metal material. Conventionally, sintering and deoxidizing are achieved in separate steps, often using separate equipment. A substantial amount of time and money associated with the processing of the valve metal could be saved by combining the sintering and the deoxidizing steps.
Accordingly, a need exists for a method of sintering valve metals to preferably achieve coarsening without densification of the sintered valve metal material that provides for retention of pore volume and surface area, a limited extent to sintering, and an increase of compact or crushed strength. Additionally, a need exists for a method of sintering and deoxidizing a valve metal material in one combined step for use in forming a capacitor having high capacitance.